The Whistling Thorn (Acacia drepanolobium).
This species of acacia grows two kinds of thorns. The main defense is provided by pairs 5cm long arranged nearly at a right angle. Smaller stipular spines grow between the pairs of big thorns. These emerge from hollow galls, bulbous swellings 2 to 3cm across. One of four different species of ant lives in each of these igloos, which they open up by cutting holes into them. A dying bush whistles as the wind blows over these entrances.
Most acacias make toxins that it rushes to leaves that are under attack by browsers. The whistling thorn does not. It is infested with stinging ants that swarm out and prepare to bite anything they can when the branch is disturbed. Most browsers seem to avoid infested bushes, perhaps because the ants stink of formic acid.

Ants: in Africa and Central Americs, symbiosis with ants can deter all sizes of enemy, from elephants to caterpillars and stem-boring beetles.[2] Some species of ants will also fight off competing plants around the acacia, cutting off the offending plant's leaves with their jaws and ultimately killing it. Other associated ant species appear to do nothing to benefit their hosts.